This invention relates to a process for the preparation of 1-benzyl-4-[(5,6-dimethoxy-1-indanon)-2-yl]methyl piperidine hydrochloride (Donepezil HCl) of the formula 1. 
Compound of the formula 1 commonly known as Donepezil HCl is used for treatment of Central Nerve System (CNS) disorders.
U.S. Pat. No. 4,895,841 describes preparation of Donepezil HCl by reacting 5,6-dimethoxy-1-indanone with 1-benzyl-4-formylpiperidine in the presence of a strong base such as lithium diisopropyl amide followed by reduction with palladium carbon catalyst (Examples 3 and 4). Overall yield of Donepezil HCl is reported to be 50.8% (62%xc3x9782%).
U.S. Pat. No. 5,606,064 teaches the preparation of Donepezil HCl by the reaction of 5,6-dimethoxy-1-indanone with pyrindin-4-aldehyde. The resulting 5,6-dimethoxy-2-(pyridin-4-yl)methyleneinda-1-one is reacted with benzyl bromide to afford 1-benzyl-4-[(5,6-dimethoxy-1-indanon-2-ylidine]methyl pyridinium bromide which on reduction with platinum oxide catalyst afforded Donepezil HCl. (Examples 2, 4 and 6). Overall yield of Donepezil HCl 58.5% (87%xc3x9783%xc3x9781%).
PCT Publication No WO 97/22584 reports preparation of Donepezil HCl by reacting pyridine-4-aldehyde with malonic acid. The resulting 3-(pyridin-4-yl)-2-propenoic acid was reduced with rodium on carbon under hydrogen atmosphere to give 3-(piperidin-4-yl)-2-propionic acid which on reaction with methyl chlorocarbonate gave 3-[N-(methoxycarbonyl)piperidin-4-yl]propionic acid. On reacting 3-[N-(methoxycarbonyl)piperidin-4-yl]propionic acid with oxalyl chloride, methyl 4-(2-chlorocarbonylethyl)piperidin-1-carboxylate is obtained which on reaction with 1,2-dimethoxy benzene in the presence of aluminium chloride afforded 4-[3-(3,4-dimethoxyphenyl)-3-oxopropyl]piperidin-1-carboxylate. On reacting 4-[3-(3,4-dimethoxyphenyl)-3-oxopropyl]piperidin-1-carboxylate with tetramethyl diamino methane, 4-[2-(3,4-dimethoxy benzoyl)allyl]piperidin-1-carboxylate is obtained which on treatment with sulphuric acid gave methyl 4-(5,6-dimethoxy-1-indanon-2-yl methyl)piperidin-1-carboxylate. On decarboxylating 4-(5,6-dimethoxy-1-indanon-2-yl methyl)piperidin-1-carboxylate, 5,6-dimethoxy-2-(piperidin-4-yl methyl)-1-indanone is obtained which on treatment with benzyl bromide afforded Donepezil HCl (Example 1 to 6). Overall yield of Donepezil HCl 19.3% (70%xc3x9784%xc3x97100%xc3x9768%xc3x9779%xc3x9761%).
U.S. Pat. No. 6,252,081 teaches the preparation of Donepezil HCl by the reaction of 1-indanone derivative with carbonate ester. The resulting 2-alkoxycarbonyl-1-indanone derivative is halogenated with (4-pyridyl)methyl or a salt thereof and decarboxylated successively to give 2-(4-pyridyl)methyl-1-indanone derivative. On reacting the 2-(4-pyridyl)methyl-1-indanone derivative with benzyl bromide, their quarternary ammonium salts are formed, which on reduction with platinum oxide catalyst gives Donepezil HCl (Examples 1 to 3). Overall yield of Donepezil HCl 82% (98%xc3x9785%xc3x97100%xc3x9799%).
The prior art processes employs 1-benzyl-4-formyl piperidine as starting material whose synthesis is low yielding and involves use of lithium diisopropyl amide. The reaction of 1-benzyl-4-formyl piperidine with 5,6-dimethoxy-1-indanone also involves use of lithium diisopropyl amide and cryogenic temperatures which are expensive and are not economically viable. Lithium diisopropyl amide is toxic and needs to be carefully handled. Many a time selective reduction of double bond to yield 1-benzyl-4-[(5,6-dimethoxy-1-indanon)-2-yl]methyl piperidine hydrochloride is difficult to achieve. Besides, the selective reduction of double bond to give 1-benzyl-4-[(5,6-dimethoxy-1-indanon)-2-yl]methyl piperidine hydrochloride is many a time coupled with formation of side products which are difficult to separate. Yield of the product is also affected due to the formation of the side products. Use of oxalyl chloride chemistry is difficult for scale up. Besides the oxalyl reaction also involves many protection deprotection chemistry and the over all yield is very low. Raw materials like methyl chlorocarbonate or tetramethyl diaminomethyl are expensive and difficult to source commercially. The prior art processes are also time consuming and difficult to carry out as they involve many steps.
An object of the invention is to provide a process for the preparation of 1-benzyl-4-[(5,6-dimethoxy-1-indanon)-2-yl]methyl piperidine hydrochloride which eliminates formation of byproducts and gives high yield of the product and is efficient and economical.
Another object of the invention is to provide a process for the preparation of 1-benzyl-4-[(5,6-dimethoxy-1-indanon)-2-yl]methyl piperidine hydrochloride which employs less number of reaction steps and is less time consuming and easy and convenient to carry out.
Another object of the invention is to provide a process for the preparation of 1-benzyl-4-[(5,6-dimethoxy-1-indanon)-2-yl]methyl piperidine hydrochloride which eliminates use of hazardous reagents and is safe to carryout.
Another object of the invention is to provide a process for the preparation of 1-benzyl-4-[(5,6-dimethoxy-1-indanon)-2-yl]methyl piperidine hydrochloride which employs cheaper and easily available raw materials.
Another object of the invention is to provide a process for the preparation of 1-benzyl-4-[(5,6-dimethoxy-1-indanon)-2-yl]methyl piperidine hydrochloride which is suitable for industrial scale up.
According to the invention there is provided a process for the preparation of 1-benzyl-4-[(5,6-dimethoxy-1-indanon)-2-yl]methylpiperidine hydrochloride (Donepezil HCl) of the formula I 
comprising hydrogenating 5,6-Dimethoxy-2-(pyridin-4-yl)methylene inda-1-one with a noble metal oxide catalyst in an organic solvent at 20-50xc2x0 C. and 10-45 psi gauge pressure to form 4-[(5,6-dimethoxy-1-indanon)-2-yl]methyl piperidine of the formula II 
which is alkylated with an alkylating agent in an organic solvent at 20-80xc2x0 C.
The organic solvent used in the hydrogenation of compound of the formula II may be tetrahydrofuran, methanol or acetic acid or combination thereof preferably acetic acid:methanol mixture.
The noble metal oxide catalyst used in hydrogenation may be platinum or palladium oxide, preferably platinum oxide.
Preferably hydrogenation is caried out at 25-40xc2x0 C. and 25-40 psi gauge
The alkylating agent may be benzyl bromide or benzyl chloride, preferably benzyl bromide.
The organic solvent used in the alkylation reaction may be methylenedichloride, triethyl amine or mixtures thereof preferably methylenedichloride and triethyl amine mixture.
Preferably the alkylation is carried out at 30-40xc2x0 C.
The process of the invention eliminates formation of byproducts and gives high yield of the product (about 92%). It employs cheaper and easily available raw materials and eliminates use of hazardous reagents. It is, therefore, efficient and economical and safe to carryout. It comprises only two steps and is, therefore, less time consuming and is easy and convenient to carryout. For the above reasons, it is also suitable for industrial scale up.